$\begingroup$What research have you done on this question so far? For instance, have you decided on a particular definition of "dead?" If you're using physics, it is well known that the cardiovascular system needs to continue operating if you want any long durations of functioning activity. This is actually a well studied topic, with many answers. Google may be your friend.$\endgroup$
– Cort AmmonAug 31 '15 at 17:16

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$\begingroup$Dead bodies are incapable of acting alive. Without an oxygenated blood flow, muscular activity cannot occur. You can create some alternate definition of 'dead' that allows for this, but most of the functions of a living body go towards allowing that body to walk around and move, or to be able to keep doing so in the future.$\endgroup$
– ckerschAug 31 '15 at 19:37

$\begingroup$Have you looked at the many fictional worlds where zombiism is presented as a non-magical disease? Like Cort Ammon says, I feel like others have probably tried this before.$\endgroup$
– zetaAug 31 '15 at 20:37

$\begingroup$Yes, but I'm looking for other explainations$\endgroup$
– TrEs-2bAug 31 '15 at 20:37

8 Answers
8

Ophiocordyceps unilateralis is a very interesting kind of parasitic fungus. It infects ants and affects their central nervous system in a way which alters the behavior of the ant. The ant stops working for the ant colony, and instead climbs the stem of the nearest plant, which is the ideal environment for the fungus to grow. The fungus then kills the ant and eats it from the inside out.

Now ants are very simple lifeforms which have no complex brains like humans. But the precedent is there. So just imagine a fungus like this which targets humans instead of ants. It infects the human brain and takes over its function to turn the human into a vessel for spreading itself to other humans.

This explanation for the zombies origin was used, for example, in game "The Last of Us".

Another common variant of this is a virus or bacterium which affects the brain function of the infected humans and changes their behavior to spread the infection.

First real zombies are people were drugged and kept as slaves by Haitian Voodoo. They would poison the person who would appear to die. Get buried and then were dug up and kept drugged as slave labor.

Now for zombies closer to the movies, the host really can't be dead. They can be brain dead, but the body must be alive. One possibility is a bacteria or fungus that causes extreme mental issues and the ability to ignore pain, maybe able to close off arteries quickly to prevent bleeding to death quickly.

Nanobots might also provide that same type thing, but a foreign intelligence (of sorts) could be replacing what was once human. Though you'd have to say that the nanobots needed the brain's pathways to control the body and when it is destroyed their control vanishes.

Last, an alien life form that uses group intelligence infects humans, but isn't completely compatible so both human and alien are in a retched state, fighting for control and the body is looking for help, the aliens trying to find a viable host, the human to get rid of the parasite, by suicide if necessary.

$\begingroup$I'd add that the external nanobots/alien life/parasites etc would need to be somehow able to sustain the host body somehow so that it can continue to "function" as a zombie. Ie providing nutrients/oxygen and other necessary sustenance to keep the muscles and bones from collapsing$\endgroup$
– NotarasSep 1 '15 at 2:27

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$\begingroup$@kapetanios yes, as I pointed out in my 2nd paragraph, the body can't be 'dead'$\endgroup$
– bowlturnerSep 1 '15 at 2:29

How about a creature (plant, animal or other) that spreads throughout the body and forces the parts to move around so that the body can bite and spread the tiny creature to new hosts? That's how I always imagined it.

“When you get bit you turn into a zombie”

Simple. Germs/mites etc. that reproduce in zombies and cause the condition also congregate in the mouth. Lots of real viruses do this.

While simple, this is not a necessary feature of classic (Romero) zombies. Arguably, they spread their condition simply by inducing fatal sepsis from a mass of mundane germs. Their condition is death itself while under the influence of some unexplained cosmic force, i.e. magic.

“The body is dead but still able to move”

It would have to be moved like a puppet. This would require the cause of zombification to digest the rotting human musculature, breathing apparatus etc. and to replace these with new and effective organs. This is unheard of in nature, and not practical for a bio-weapon.

I would argue that the body does not have to be dead. Settle for brain death or a debilitating change in temperament à la 28 Days Later (2002). The mites/germs rebuild parts of the brain to control the body, causing typical zombie hygiene as a side effect.

“Zombies can only be killed with a head shot”

If head shots are highly effective, that would be explained by the mites/germs using the brain in the manner of cordyceps fungi.

If all other damage is less effective than it would be against a living human, that would be explained by the destruction of relevant nerves in the zombification process to eliminate pain, coupled with enough endocrinological mayhem to push the body to brief peaks of hyperactivity in search of food, new hosts etc., but a shot through the heart would still stop a zombie.

A less credible disease vector could conceivably combat infections and other mundane causes of death to keep its host mobile, and clot blood better than we do, and so on. A still-less credible vector could repair its host. An incredibly fancy bio-weapon could build a redundant heart.

Imagine a nearby future where humanity has wired itself in order to better connected to the internet. Instead of needing a bulky, inconvenient computer, the signals would be transmitted directly through microfilimant cables under the skin. You would not need a credit card. Instead you could just lay your hand on an item and buy it immediately as the RF ID tag interacted with the tiny chip in your brain.

Now imagine something went horribly wrong, a computer virus, perhaps, or just a EMP pulse that fries the circuitry in the brain, leaving only aggression and very basic memory functions. The new and shiny technology might turn us all into zombies, effectively lobotomizing us into aggressive automatons.

The life span of these lobos would be minimal, since they would just be damaged normal people, but lets say there's an extreme adrenal response which is off the charts, so while the threat is relatively short lived, while it is active, you might have a 28 days later scenario.

The Root problem with anything except for maybe funguses is they all require a functioning body [Circulatory system, Digestive system, Respiratory System, Excretory Systems]. Without which the body simply wouldnt move and the ailment would have difficulty spreading. Not to mention in all natural cases of foreign organisms they simply do a few things to achieve their goal mostly because they have limited 'knowledge' of their host. However, this NEW concept has a few workaround to increase plausibility:

Transmittable Brain Tumors

In the case of the Tasmanian Devil, Tumors were transferred via bites and caused horrific and ultimately lethal symptoms.

However there are cool aspects about this concept that make things more possible:

As incurable as cancer

immune systems are irrelevant

greater 'knowledge' of host

certain tumors have be known to replicate other human constructs like teeth

it's a runaway mutation, which means it can be the cause to all kinds of other
mutations in the body (EX: if you wanted your zombies to sprout wings this would be the way to scientifically justify it)

mutates rapidly

Tumors can also produce all kinds of substances similar to bacteria.

So what if this transmittable tumor infiltrates the brain and rewires it to all the desired behaviors discovered in modern psychology.
What if it could spread to the body and cause all kinds of desired or undesired physical manifestations.

This too requires a functioning body, however with this there can be a cool hack around that gets closer to the appearance of death.
After the tumor invades the brain it could then spread to the body and start duplicating organs. This could make your zombies more resilient and closer to the 'undead'ness of the mythical zombie.

Key Constraints

Evolution: while cancer is a rapidly evolving illness it does follow the rules of natural selection. So in the case of zombies developing wings you would have to create a fairly long historical progressive lineage of zombies that slowly develop the traits for wings, with an ideal some making it and the less ideal some not making it. The host must also survive long enough to pass on this condition.

Metabolism: cancer can work very fast because it can develop an increased metabolic/reproductive speed, however this requires nutrition (hunger and working organs). This increased metabolic speed can be stretched for rapid regeneration. So in this instance a zombie could take apparent fatal injuries however still rise back if provided a ~day and nutrition. This falls apart if the zombie is riddled with bullets and loses too much blood thus starving all the cells. This could also explain fast and unrelenting zombies.

Starvation: an increased metabolism requires increased food intake without which the zombie would starve. It could be possible however, for the tumor to be 'smart' enough that when it senses a reduced nutrition supply to enter a dormant state halting growth/reproduction and significantly reducing the physical need for food. This would then justify slow, lethargic zombies playing possum awaiting stimuli that would indicate food. However any unhealed fatal wounds would potentially be fatal at this point.

Water/disease: Since the zombie is still 'alive' it still needs water somehow and this also bares the question of disease. The cancer is still 'human' cells which are still potentially susceptible to human infections. It's a greater stretch but it could be that it develops a mutation that alters the immune system.

The zombie slave spiders tended to build a particular kind of web, one
that was quite different from the webs created by parasitoid-free
spiders, the researchers found. First, the parasite-ridden spiders
took apart their old webs (some even abandoned them altogether), and
then they started building new ones that resembled the web an orb
weaver would build if it were about to molt, or shed its exoskeleton
(something spiders do in order to grow).

In mammals, the Rabies Virus causes reactions such as hallucinations and sometimes aggression. Perhaps it could one day evolve to induce more and more such aggression by manipulating the chemical transfers in the brain?

Actually, it's a lot more plausible then you may think. There are some diseases that share the same way of transportation: Saliva or biting. There are also some diseases that affect humans, making their skin deteriate. So, in other words, if a zombie virus did (and they might very well) exist,they could be a type of virus or fungus (there is a type of fungus that can attach themselves to ants) that either transmits by saliva or bites. It's scary to think, but there is a very real possibility of a zombie existing.